Combined magnetic shielding and beam centering assembly for cathode-ray tubes or the like



April 18, 1961 H. S. VASILEVSKIS COMBINED MAGNETIC SHIELDING AND BEAMCENTERING ASSEMBLY FOR CATHODE-RAY TUBES OR THE LIKE Filed Aug. 15, 19582 Sheets-Sheet 1 NflE/Vff 1 05/ 77 ON 19f GAEE 5 INVENTOR. HEN/F) .5.VHS/LEVf/f/ April 1961 H s. VASILEVSKIS 2,980,814

COMBINED MAGNET I0 SHIELDING AND BEAM CENTERING ASSEMBLY FOR CATHODE-RAYTUBES OR THE LIKE Filed Aug. 15, 1958 2 Sheets-Sheet 2 1N VENTOR. HAW/9)0'. mm EVjK/S United States Patent COMBINED MAGNETIC SHIELDING AND BEAMCENTERING ASSEMBLY FOR CATHODE-RAY v TUBES OR THE LIKE Pa., assignor toPhileo Philadelphia, Pa., a corporation of Penn- The present inventionrelates to television display systems and more particularly toimprovements in beam shielding and centering devices for such systems.

The present trend in television systems is to reduce the over-all lengthof the television picture tube, first by increasing the maximumdeflection angle of the beam, thereby to reduce the length of the bulbfor a given screen size, and secondly by reducing the length of the neckof the cathode-ray tube as much as possible. In reducing the length ofthe neck it becomes necessary to move the cathode and first and secondgrids of the electron gun closer to the deflection yoke. This hasintroduced the need for means for shielding these elements of theelectron gun from the effects of the fringing field of the deflectionyoke.

The reduction in the length of the neck of the cathoderay tube has alsomade it impossible to locate conventional magnetic centering devices onthe neck of the cathode-ray tube between the electron gun and thedeflection yoke. The removal of the centering devices from the neck ofthe cathode-ray tube has made it necessary to find other means forpositioning the point of impingement of the undeflected cathode-ray beamat the desired spot on the picture tube screen. Attempts have been madeto increase the precision of manufacture of cathode-ray tubes so that nocentering device is required. This greatly increases the unit cost ofacceptable picture tubes. Other attempts have been made to rely onelectrical corrective circuits associated with the yoke to providecentering. Another alternative which has been tried is to provide someform of magnetic centering devices within the yoke itself. Each of theselast two alternatives has its disadvantages and generally adds to thecost of the picture tube and the deflection yoke.

The copending application of Thomas V. Di Paolo et 211., Serial No.744,121, filed June 24, 1958, now US.

Patent No. 2,926,272, discloses a laminated shielding and centeringmember which may be placed on the neck of the cathode-ray tube at theend of the deflection yoke. This novel laminated shielding member, whilerequiring very little space on the neck of the cathode-ray tube,effectively reduces the fringing flux and provides centering in onedirection.

It is an object of the present invention to provide an improvedlaminated shielding member which requires very little space on the neckof the cathode-ray tube and which performs the dual function of magneticshielding and beam centering in two directions.

it is a further object of the present invention to provide a novelon-the-neck beam centering means which is located within theelectrostatic focusing region and which controls the position of thebeam in two directions.

Still another object of the present invention is to provide a noveltwo-direction beam centering arrangement which is external of thedeflection yoke and yet does not require any substantial amount ofadditional space on the neck of the cathode-ray tube.

These and other objects of the present invention are achieved byproviding a laminated magnetic shielding assembly comprising a highlyconductive non-magnetic sheet-like member and two spaced sheet-likemembers of high permeability secured thereto. Each high permeabilitymember is formed with a gap therein dividing that high permeabilitymember into two magnetically separate portions. The gaps in the two highpermeability members are disposed approximately at right angles to oneanother. An adjustable magnet is secured to said laminated magneticshelding member in a position to bridge the gap between the twomagnetically separate portions of the first high permeability member. Asecond adjustable magnet is secured to said laminated magnetic shieldingassembly in a position to bridge the gap between the two separateportions of the second high permeability member. Control of beamcentering is accomplished by positioning the magnets to vary thestrength of the magnetic fields impressed between the two magneticallyseparate portions of each of said two members of high permeability.

For a better understanding of the present invention together with otherand further objects thereof reference should now be made to thefollowing detailed description which is to be read in conjunction withthe accompanying drawings in which:

Fig. 1 is a view partially in section of a cathode-ray tube anddeflection yoke assembly incorporating the novel centering device of theinvention;

Fig. 2 is a graph showing the reduction in fringing flux which may beaccomplished through the use of the novel shielding and centering deviceof the present invention;

Fig. 3 is a view of the combined shielding member and centering deviceof the present invention taken in a (iirection parallel to the axis ofthe cathode-ray tube;

Fig. 4 is a cross-sectional view of the combined shield ing member andcentering device of Fig. 3 taken along the line 44 of Fig. 3;

Fig. 5 is an exploded view of the shielding device of Figs. 3 and 4; and

Fig. 6 is a plot showing the relationship between the position of thecentering magnets and the static deflection of the cathode-ray beam.

Turning now to Fig. 1, member 10 is the neck portion of a cathode-raytube on which the deflection yoke assembly incorporating the presentinvention is mounted. The neck flare of the cathode-ray tube is shown at12. The end turns of one of the horizontal deflection coils are shown at14 and 14a. One vertical deflection coil is shown at 16. An insulatingcoil form 18 separates the horizontal deflection coils 14 and 16 andserves to maintain these coils in their proper position on the neck 10of the cathode-ray tube. Member 20 represents the magneticyoke whichserves as the external magnetic path for .the flux set up by thehorizontal and vertical deflection coils.

In the embodiment of the invention shown in Fig. 1 the shielding andbeam centering assembly 28 includes an insulating housing 363 whichprovides mechanical support for the shielding and beam centeringelements. As will be shown presently, housing 30 also serves as anonmagnetic spacing member between two magnetic members of the centeringdevice. Housing 30 may also act as an enclosing means for the terminalsof the deflection coils (not shown) and for this purpose it is providedwith an annular flange 30a.

Shielding of the high frequency horizontal deflection field is providedby a highly conductive disc 32 which is disposed adjacent the end turnsof the horizontal deflection coils 14a. Shielding of the lower frequencyvertical deflection field is provided by a high permeability member 34which is disposed between highly conductive member 32 and housing 30.Centering in the vertical direction is accomplished by providing anadjustable magnet 36 which bridges a gap that separates the highpermeability shielding member 34 into two separate magnetic portions. Inthe embodiment shown in Fig. 1, magnet 36 is secured to the shieldingand centering assembly 28 by means of a rivet 38. Magnet 36 is rotatablymounted on rivet 38 thereby providing means for adjusting the verticalcentering field.

Centering in the horizontal direction is accomplished by providing asecond sheet-like high permeability memher 4% and a second adjustablemagnet 42. High permeability member 4% is spaced from the highpermeability member 34 by the non-magnetic housing 39. Magnet 42 ispivotally mounted in a position bridging the gap e4 between the twosections of high permeability member 40. Support for magnet #32 isprovided by rivet 46. As shown more clearly in Figures 3, 4 and 5,sheet-like memer 4! is so formed that it does not come in contact withmagnet 36. Also, high permeability member 34 is shaped so as to reducethe shunting effectof this member on the field induced by magnet 42.

A dielectric member 43 may be interposed between the end turns Ma of thedeflection coils and the highly conductive member 32. Dielectric member48 prevents accidental electrical contact between the highly conductivemember 32 and the end terminals or lead wires from the deflection coilcircuits. It also provides a slight spacing between the horizontaldeflection coil 14a and conductive shielding member 32. This spacingreduces the shunting elfect of the shield member on the high frequencyhorizontal deflection field.

Turning now to Figs. 3, 4 and for a more complete description of thenovel shielding and centering assembly 28 shown in Fig. 1, it will beseen that the conductive disc 32 which is disposed closest to thedeflection coils is a circular disc provided with an aperture 50 forreceiving the neck portion of the cathode-ray tube. Conductive disc 32is provided with a number of small holes 52 for receiving rivets 38 and46 which support vmagnets 36 and 42, respectively, and to receiveadditional rivets which are employed in one commercial embodiment of theinvention to secure the elements 32, 34 and 40 to housing 30.

The high permeability member 34 is formed of two half discs 34a and 34bwhich are separated by a nonmagnetic gap 56. The central region 58 ofthe gap is enlarged to provide an aperture for receiving the neckportion 10 of the cathode-ray tube. The half discs 34a and 34bareformed. with holes 6% which register with the holes 52 in conductivemember 32 to receive the rivets mentioned above. an aperture 62 which islocated adjacent magnet 42 on member 49. The outline of aperture 62 isshown by the broken line 62 in Fig. 3. The purpose of aperture 62 is tominimize the shunting efiect of half disc 34 on the gap 44 in member 40.In other embodiments of the invention the aperture 62 may extend to theouter edge of member 34a and thus form a notch in this outer edge.Although no aperture or notch is required in member 34b for properoperation of the invention, member 34b may be shaped the same as member34a in order to reduce the cost of construction of the assembly 28.

As shown in Fig. 3, magnet 36 is pivoted so that it bridges the gapEdbetween half discs 34a and 34b. Magnet 36 is magnetizedalong adiameter as indicated by the letters N and S in Figs. 3 and 5. Theletters N and S designate the north and south poles of the magnet 36. Itwill be seen that rotation of magnet 36 changes the net magnetic fluximpressed between the two halves 34a and 34b of shielding and centeringmember 34. Obviously other equivalent means may be provided forsupporting magnet 36 and magnet 36 may have shapes other than circularand still accomplish the same result. For example, magnet as may takethe form of a simple bar magnet which is held in place by its ownmagnetism. If a more permanent attachment is required the magnet canHalf disc 34:: is formed With,

be adjusted until the proper centering is achieved and then secured inplace by a suitable adhesive.

Housing 36 is provided with an aperture 66 for receiving the neckportion til of the cathode-ray tube. A portion of housing 3% is also cutaway as shown at 8 to receive the magnet 36. Since magnet 36 projectsthrough the cut-away portion 68 of housing 3%, it may be readilyadjusted without demounting the shielding and centering assembly of thecathode-ray tube. Again the holes 70 formed in housing 30 are providedfor receiving the rivets.

As shown in Figs. 3 and 5, high permeability member 40 is formed of twosimilar portions 4% and 49b. Members 40a and till; are shown as havingthe some shape. Although this is not essential to the proper operationof the invention, it is desirable from a manufacturing standpoint sinceit minimizes the number of parts of different shapes that are required.Members and itlb are formed with a generally circular edge The edgeportion 74 of member dim is so formed that her 4% does not overlie themagnet 36. The finger-like extensions 76 and '73 of members 40:! and40b, respectively, encircle the neck portion 13 of the cathode-ray tubeand terminate in a gap 30 which may be somewhat larger than the gap 4-4.The reduced cross-sectional of finger-like portions 76 and 78 minimizesthe shunting effect of member 49* on the gap 56 in member 3%. As shownin Figs. 3 and 4, sheet-like members and 3617 are secured to therearward surface of housing 3t. Therefore member 49 is spaced from themember 34 by the thickness of non-magmetic housing 3%.

As mentioned in the description of Fig. 1, magnet 32 is so mounted thatit bridges the gap between magnetically separate members 4012 and Magnet42 is also magnetized along a diameter as represented by the letters Nand S which designate the north and south poles, respectively, I" themagnet. As mentioned in connection with the description of magnet 35,magnet may have a shape other than circular and be held in position bymeans other than rivet 46.

The laminated shielding and centering assembly shown in the drawingsprevents fringing oi the horizontal and vertical deflection fields inthe following manner. The eddy currents set up in the highly conductivememoer 32 by theapproximately 15 kilocycle horizontal scening field tendto confine the flux from the end regions of. the horizontal deflectioncoils 14a to the region of neck portion it to the left of the assembly23 as shown in Fig. 1. The shielding enect or" member 32 prevents thehigh frequency horizontal deflection flux from reaching the highpermeability members 34 and 4%. Therefore the novel laminated shield ofthe present invention shunts much less of the high frequency horizontaldeflection field around the neck of the tube than would be shunted by asingle layer shield of high permeability material at the same location.Member 32 also aids in reducing the hysteresis losses in member 34 byreducing the concentration of high frequency magnetic flux in this highpermeability member. The reduction of the shunting effect and thereduction of the hysteresis loss both tend to minimize the loss ofhorizontal deflection power in the shielding assembly 28.

The eddy currents induced in conductive disc 32 as a result of therelatively low frequency vertical scanning field are usually notsuificient to confine the vertical field to the region to theleft ofdisc 32. Therefore some of the fringing flux passes through conductivedisc 32 to the low'reluctance members 34a and 34b. This fringing flux isconducted around the market the cathoderay tube through members 34a and34b. The amount of this shunted fiux may be partially controlled byproperly selecting the size of the gap 56 in the otherwise lowreluctance path afiorded by members 34a and 34b. In general, the size ofthis gap 56 will represent a compromise between the amount of defocusingwhich can be:

tolerated at the edges of the picture tube as a result of residualfringing flux and the permissible loss of vertical deflection powerresulting from the shunting effect of the shielding means. In onetypical embodiment of the invention gap 56 has a width of It has beendetermined experimentally that the shunting effect of the laminatedshield member 28 on the low frequency deflection field can be changedmeasurably by including gap 56. This is so even though there are otherrelatively large air gaps in the path of the vertical deflection flux.

Fig. 2 is a plot of the field strength of the deflection flux along theaxis of neck portion it? of the cathode-ray tube. The horizontal scaleis in units of distance from the end plane of the deflection coils. Theposition of the control grid of the cathode-ray tube of Fig.1 is shownat 84 as a convenient reference point in evaluating the curves. Curve 86represents the field strength without the assembly 28 in place. Curve 88represents the field strength with the assembly 28 in place. It shouldbe noted that the amount of fringing flux at the control grid region 84has been reduced by a factor of approximately 3. At the same time thefield strength at the end of the deflection coils is reduced onlyslightly.

Fig. 6 is a plot of the change in the static beam position which may beachieved by rotating magnet 36. The zero or reference direction is takenwith the north-south axis of magnet 36 aligned with the gap 56, that is,with a diameter of disc 34. Rotating the magnet 36 from this referenceposition places the north-south axis across the gap 56. As a resultthere is a steady flux which flows through half discs 34a and 34b andbridges the gap between the adjacent edges of these half discs. Some ofthis steady flux also fringes between the arcuate edges of the aperture58, that is, across the region occupied by the neck portion 10 of thecathode-ray tube. This flux which fringes across the region occupied bythe neck portion 10 of the cathode-ray tube causes a deflection of thecathode-ray beam in the vertical direction. The amount of deflection inthe vertical direction will increase as the angle between thenorth-south axis of magnet 36 and the gap 56 increases. The maximumdeflection, as represented by the peak 92 in Fig. 4, is reached when thenorth-south axis is perpendicular to the reference direction, that isperpendicular to the gap 56. Further rotation of the magnet 36 willreduce the amount of magnetic flux induced in half discs 34a and 34b.This reduction in flux will reduce the vertical deflection of the beam.The deflection of the beam from its normal rest position will again be aminimum when the north-south axis of magnet 36 again moves intoalignment with the gap 56. If the magnet is rotated so that the northpole is now on the opposite side of the gap from the position justdescribed, the deflection will again be in the vertical direction but inthe opposite sense as represented by the negative peak 94 in the curveof Fig. 4. That is, if the beam is directed downwardly with the northpole of magnet 36 in contact with half disc 34a, the beam will bedeflected upwardly if the north pole is moved into contact with the halfdisc 3-4:). The misalignment of the beam-spot on the screen of thecathode-ray tube is usually caused by misali nments between the gunassembly and the neck of the cathode-ray tube rather than bymisalignments in the gun assembly itself. Therefore centering of thespot on the screen on the cathode-ray tube may require an intentionalmisalignment of the beam with the axis of the gun. For this reason it isdesirable to locate the region in which centering occurs as far forwardon the electron gun assembly as possible. This location of the centeringregion minimizes the linear distance by which the beam is displaced fromthe axis of the focusing assembly by the centering means. Too great adisplacement of the beam may place it in a region of poor focus and maycause the beam to strike elements of the gun or the neck of thecathode-ray tube. If the beam strikes an element of the gun or the neckof the cathode-ray tube it will cause shadowing of the picture on thescreen. The novel shielding and centering device of the presentinvention places the centering region much further forward than ispossible with conventional on-the-neck centering devices.

Since the only flux that is effective in positioning the beam is theflux that fringes across the neck portion 10 of the cathode-ray tube,the width of gap 56 must be selected with reference to the strength ofmagnet 36 to provide the necessary flux across the neck region of thecathode ray tube. a

The effective width of gap 56 is affected somewhat by the presence ofthe finger-like portions 76 and 78 of member 40. Finger-like members 76and 78 of sheet-like member 40 are spaced from members 34a and 3411 bythe non-magnetic housing 30. This reduces the shunting effect of thesefinger-like portions 76 and 78 on the gap 56. Nevertheless it isdesirable to make the width of portions 76 and 78 as small as possibleconsistent with proper centering in a horizontal direction in order tostill further minimize the shunting action of the sheetlike members 40aand 40b. Proper centering in the horizontal direction will usuallyrequire a cross-sectional area of finger-like portions 76 and 78 whichis large enough to prevent saturation of these portions by the flux frommagnet 46.

The presence of the aperture 62 in disc-like member 34a-minimizes themagnetic flux induced in member 340 by the magnet .42. Furthermore anyflux induced in sheet-like member 34a by magnet 42 will tend to fringecross the neck portion 10 in a vertical direction as viewed in Fig. 3,rather than in a horizontal direction. Therefore this flux from magnet42 will have very little effect on the vertical centering of thecathode-ray tube. Any effect introduced by the adjustment of magnet 42can be compensated for by minor readjustment of the magnet 36.

Control of the position of the spot in the horizontal direction isachieved by adjusting the position of magnet -42. Rotating magnet 42about its support 46 will cause the beam to change in position in thehorizontal direction in a manner similar to that shown in Fig. 6. Theactual shape of the curve may vary somewhat from that shown in Fig. 6owing to the differences in the shape of sheetlike members 40a and 34a,for example. The shape of the curve will also be affected by the largergap at fail.

Nevertheless the curve will have the same general shape,

increasing to a maximum in one direction as the axis of magnet 42 isrotated to a position perpendicular to the gap 44, then decreasing tozero and increasing to a maximum in the opposite direction as magnet 42.is rotated through 180. The relatively large cross-sectional area ofmembers 40a and 40b to the left of the figure as shown in Fig. 3provides an adequate area of contact between members 4041 and 46b andmagnet 42. This enlarged portion also permits the supporting rivets tobe relatively widely spaced. This tends to reduce the variation in thesize of gap 80 which might result from minor changes in location of theholes which receive the supporting rivets. Gap S0 is made relativelylarge so that any small variation in the position of the ends offinger-like portions 76 and 78 which might occur during mass productionassembly do not make an appreciable percentage change in the length ofthe air gap 80. increasing air gap 80 also tends to increase the amountof fringing flux which passes through the region occupied by the neckportion 10 of the cathode-ray tube. The shape of finger-like portions 76and 73 and the length of the air gap 8t} will control in some degree thedistribution of the fringing flux across the area occupied by the neckportion 10 of the cathode-ray tube. The shapes of finger-like portions76 and 78 and the length of gap 86 may be selected so as to tend toprovide a uniform distribution of the flux from magnet 42 which fringesacross the area occupied by the neck region 10 of the cathoderay tube;If there is a uniform distribution ofthis flux across the neck' regionoccupied by the neck portion 16, the presence of magnet 42 and thesheet-like members 4th: and 4th; will control the position of the spotbut will have no airect on the linearity of the travel of the spotacross the screen of the cathode-ray tube. Alternatively, the shape offinger-like portions 76 and 78'and the size of 86) may be purposelyselected to provide a nonuniform distribution ofsteady magnetic fieldacross the region occupied by the neck portion 10: This intentionallyintroduced nonlinearity may be employed to correct for nonlinearities inthe horizontal sweep voltage generation circuit.

Again adjustment of magnet 36 shouldhave little or no etfect on theoperation of the horizontal centering means represented by magnet 42 andsheet-likemembers itia and 49b. However if the magnet does have a slighteifect on the adjustment of the horizontal position of the beam, thischange may becompensated for by a slight readjustment of the magnet 42.

Various changes and modifications may be made in the preferredembodiment shown which are within the scope of the present invention.For example the aperture 53 in member 34 may be given a shape other thancircular in order to control the distribution of the flux from magnet 36which fringes across the region occupied by the neck of the cathode-raytube; Theshape of gap 56 may be changed for the same reason. Members 32,34 and 4% have been shown as fiat plates. It-lies'within the scope ofthe present invention to dish these members slightly so that theyconform more closely to the end of the defiection yoke.

While the invention has beendescribed with reference to a singleembodiment thereof, it will be apparent that various modifications andother embodiments thereof will occur to those skilled in the art withinthe scope of the invention. Accordingly I desire the scope of myinvention to be limited only by the appended claims.

What is claimed is:

1. In combination with a cathode-ray tube system having magneticdeflection coils for creating a relatively high frequency magneticdeflection field in a first plane and a relatively low magneticdeflection field in a second plane, a laminated magnetic shielding andbeam centering assembly comprising a sheet-like member formed of ahighly conductive material, said sheet-like member being apertured toreceive the neck of said cathode-ray tube and disposed in adjacency withsaid defiection coils, a first pair of semicircular coplanar plates ofhigh pero is:

meability material disposed in juxtaposition with said sheet-like memberon the-side of said sheet-like member remote from said coils; selectedportions-ofthe diametrical edge of one of said first pair of platesbeing disposed in spaced juxtaposition to corresponding portions of theother plate of said first pair thereby to define agap pair of plates andan integrally formed, arcuate, finger like portion of relatively smallradial dimension extending partially around the neck of said cathode-raytube, a selected portion of an edge of said main portion of one of saidplates in said second pair being disposed in spaced juxtaposition to acorresponding portion of the other plate in said second pair thereby todefine a second gap extending transversely to said first gap, a secondpermanent magnet bridging said second gap between said second pair ofsaid coplanar plates, said second magnet being so oriented as toestablish a magnetic field between said second pair of said coplanarplates and across the neck of said cathode ray tube, one of said platesof said first pair being formed with an aperture therein underlying saidgap between said second pair of plates, saidsecond magnet extendingthrough said last-mentioned aperture in a non-contacting relationship.

2. A laminated magnetic shielding and beam centering assembly as inclaim 1, wherein the ends of said arcuate, finger-like portions define athird gap wider than said second gap and in line therewith.

References Cited in the file of this patent UNITED STATES PATENTS2,634,381 Kafka Apr. 7, 1953 2,653,262 Bowman Sept. 22, 1953 2,717,323Clay Sept. 6, 1955 2,761,989 Barkow Sept. 4, 1956 2,813,212 GrundmannNov. 12, 1957 2,817,782 Over Dec. 24, 1957 2,860,329 Reiches Nov. 11,1958

